1. Field of the Invention
The present invention relates to a mobile communication system and a communication control method for use with the system including cells of which each defines a predetermined radio wave arrival range of radio waves from a fixed base station associated with the cell, and in particular, to a mobile communication system and a communication control method for use with the system suitable for a mobile unit or module such as a train moving at a high speed.
2. Description of the Prior Art
To guarantee wide uses of mobile communication terminal devices using radio or wireless communication, communication enterprises have been endeavoring to develop areas for the terminal devices. Recently, the areas have been broaden beyond indoor areas to include zones such as places in a subway train and a tunnel. The terminal device can be used in a moving train and even in a high-speed mobile unit such as a train of the Japanese rapid line or Shinkansen today. Therefore, it is expected that many people uses such communication terminals in a moving train. For this purpose, a type of network business is being developed using communication terminals in a moving train. At present, it has been desired to develop a system in which the terminal in a car moving at a high speed can continuously keep a communication line to a party of communication.
For example, Japanese Patent Laid-Open No. Hei 11-298967 filed before the present specification describes a technique in which a wireless mobile station which can be connected to a public communication line and/or a leased communication line to communicate with an outer-vehicle wireless base station disposed outside a vehicle and an inner-vehicle wireless base station to communicate with the mobile station and a wireless terminal device in a vehicle are arranged so that signals are transmitted from the vehicle to the outer-vehicle base station.
Moreover, Japanese Patent Laid-Open Ser. No. 09-327057 proposes a mobile communication system in which to enable communication in a running train by use of a mobile terminal device (PS) for a micro-cell system, there are disposed a plurality of inside wireless stations (CS) in a train for a micro-cell system to communicate with the mobile terminal so that a wireless base station (BS) disposed outside of the train is connected to the inside stations (CS).
Japanese Patent Laid-Open No. Hei-10-164640 describes a mobile communication system in which a communicating device to communicate with a mobile terminal in a mobile unit is arranged in a place of a first end thereof and in a place of a last end thereof. When the mobile unit moves from a first position to a second position, the communicating devices at the first and last ends of the mobile unit conduct a change-over operation with respect to an external fixed base station to continuously retain a linked state in which each of the communicating devices establishes a link with the fixed base station.
In the conventional cell configuration of the fixed wireless base stations, cells are adjacent to each other or overlap with each other to continuously use a communication line between a mobile wireless device and a fixed wireless station.
As can be seen from FIG. 16, for example, when a mobile wireless device 100 in a linked state for a fixed wireless base station A, namely, in a state capable of communicating with the base station A moves in a running direction to a position outside a cell a configured by the base station A, the device 100 enters a non-linked or disconnected state for the base station A, i.e., a state in which the device 100 cannot communicate with the base station A. That is, the link between the wireless device 100 and the base station A is disconnected and hence communication is not possible therebetween. To continuously keep the communication of the wireless device 100 in this situation, the system is configured so that the cell a of the base station A is contiguous to a cell b constructed by a fixed wireless base station B or the cell a overlaps with the cell b as shown in FIG. 17. Thanks to this configuration, the link between the mobile device 100 and the base station A is released and a new link is prepared between the device 100 and the base station B in the situation described above. As a result, the communication line being used by the wireless terminal device 100 is kept connected.
However, as can be seen from FIG. 17 showing the cell configuration of the prior art, the cells (a and b) are adjacent to each other or overlap with each other in consideration of each size of a radio signal receivable range of each fixed wireless base station and a distance between the base stations. That is, the cells are configured only according to the fixed base stations. To reduce the number of base stations in this case, it is favorable to separate the cells (a and b) from each other as shown in FIG. 18. However, in accordance with Japanese Patent Laid-Open Nos. Hei-11-298967 and Hei-09-327057, when the cells (a and b) are apart from each other, a non-communicable zone in which communication is not possible between a fixed wireless base station and the mobile wireless device appears between the cells a and b. As a result, the mobile wireless device 100 cannot continuously conduct communication.
In the technique described in Japanese Patent Laid-Open No. Hei-10-164640, even when the cells (a and b) are separated from each other as shown in FIG. 18, it can be considered that the first and second mobile communication units become alternately linked with fixed wireless base stations to continuously conduct communication. However, as implied by FIGS. 1 to 5, the technique has an object in which the first and second mobile communication units are alternately set to the linked state, namely, either one there of is in the communicable state to continuously carry out communication when the cells overlap with each other. When either one of the mobile communication units enters a linked state, a switching section releases the linked state of the other one thereof into a non-linked state so that only one of the communication units is in the linked state. This complicates control processing of the switching section and hence there exits a fear of delay in the switching operation to establish a linked state in the mobile communication unit.